Digital electrochromic mirror system

ABSTRACT

An electrochromic rearview mirror system for a vehicle includes an electrochromic reflective element having an electrochromic cell, wherein the reflective element colors to a partial reflectance level in response to a drive signal applied to the cell. The rearview mirror assembly additionally includes a drive circuit which applies a pulsed drive signal to the electrochromic cell in order to establish the partial reflectance level of the reflective element. The drive circuit controls the partial reflectance level as a function of the duty cycle of the pulsed drive signal, which has a pulse repetition rate of at least approximately 10 cycles per second and preferably at least approximately 20 cycles per second. The drive circuit additionally adjusts the amplitude of the pulses as a function of the voltage developed across the electrochromic cell.

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to vehicle rearview mirrorsystems and, more particularly, to electro-optic mirror assemblies, suchas electrochromic rearview mirror assemblies for a vehicle.

[0002] Electrochromic rearview mirror assemblies include anelectrochromic reflective element made up of a reflecting surface and anelectrochromic cell positioned between the driver and the reflectingsurface. The electrochromic cell responds to a direct current (DC)voltage applied across a pair of terminals by varying the lighttransmittance through the cell. In this manner, the reflectance level ofthe reflective element can be varied by varying the DC voltage appliedto the electrochromic cell. The electrochromic cell has characteristicswhich make control of the reflectance level of the reflective elementdifficult. The electrochromic cell operates at a relatively low voltage,typically which may not exceed approximately 3 volts DC, more typicallynot more than about 1.5 volts DC, for more than a brief period of timeor else useful life of the reflective element is compromised.Furthermore, the amount of drive current necessary to color or bleachthe cell varies both with the temperature of the cell and the amount ofchange in light transmittance undertaken. Therefore, optimum control ofthe electrochromic cell requires more than merely applying a DC voltagecorresponding to the desired reflectance level.

[0003] One approach to controlling the reflectance level of anelectrochromic cell is disclosed in commonly assigned U.S. patentapplication Ser. No. 08/768,193 filed Dec. 17, 1996, by the presentinventor and Niall R. Lynam, entitled AUTOMATIC REARVIEW MIRROR SYSTEMWITH AUTOMATIC HEADLIGHT ACTIVATION. In this co-pending application, theelectrochromic cell is driven by an analog feedback system whichtranslates a desired reflectance level, produced by an analog circuit,to a signal applied to the electrochromic cell which drives the cell tothe desired reflectance level. While such drive system is effective, itrequires the use of analog components. Such analog components would beredundant in a digital electrochromic mirror system and, therefore,would unnecessarily add to the cost of the system. However, substitutionof digital components for the previously used analog components is not astraightforward matter. Digital components typically operate betweendiscrete output states which may include binary devices, such astransistors, switches, and the like, which exhibit a low and a highstate, and tristate devices, such as types of microprocessors whichexhibit a neutral. a low, and a high state. Such components are usefulin processing data but are not readily adapted to controlling thereflectance level of an electrochromic rearview mirror. In particular, atypical electrochromic mirror utilized as an interior mirror of avehicle may have a surface area in the range of 90 cm² to 150 cm² andtypically in the range of 110 cm² to 130 cm². A steady state currentdraw, after color transitions have settled, is typically in the range ofbetween approximately 60 milliamperes and 180 milliamperes with a rangeof 80 milliamperes to 150 milliamperes being typical. Exterior rearviewmirrors can be even larger with a surface area of approximately 350 cm²,and greater, and a commensurate increase in current density.

SUMMARY OF THE INVENTION

[0004] The present invention provides a digital electrochromic mirrorsystem which utilizes primarily digital components to drive anelectrochromic cell of an electrochromic mirror system to a desiredreflectance level which not only meets, but desirably exceeds theperformance of prior analog systems.

[0005] According to an aspect of the invention, an electrochromicrearview mirror system for a vehicle includes an electrochromicreflective element having an electrochromic cell wherein the reflectiveelement colors to a partial reflectance level in response to a drivesignal applied to the electrochromic cell. The rearview mirror assemblyadditionally includes a drive circuit which applies a pulsed drivesignal to the electrochromic cell in order to establish the partialreflectance level of the reflective element. The drive circuit controlsthe partial reflectance level at least as a function of the duty cycleof the pulsed drive signal.

[0006] According to another aspect of the invention, an electrochromicrearview mirror assembly for a vehicle includes such an electrochromicreflective element and a drive circuit which applies a drive signal tothe electrochromic cell in order to establish the partial reflectancelevel of the reflective element. The drive circuit includes a digitalcontroller, a binary switching device responsive to an output of thecontroller for applying a source to the electrochromic cell, and aninput of the controller. The input of the controller is preferablyresponsive to the voltage developed across the electrochromic cell bythe source. The digital controller closes and opens the binary switchingdevice according to a particular duty cycle in order to control thepartial reflectance level at least as a function of the duty cycle. Thedigital controller additionally adjusts the source as a function of thevoltage developed across the electrochromic cell.

[0007] According to yet an additional aspect of the invention, anelectrochromic rearview mirror assembly for a vehicle includes such anelectrochromic reflective element and drive circuit which applies adrive signal to the electrochromic cell in order to establish thepartial reflectance level of the reflective element. The drive circuitincludes a digital controller, a first binary switching deviceresponsive to an output of the controller for applying a source to theelectrochromic cell, and a second binary switching device which isresponsive to an output of the controller for draining charge from theelectrochromic cell. The controller alternatingly closes the switchingdevices according to a particular duty cycle in order to control thepartial reflectance level as a function of the duty cycle. The digitalcontroller closes and opens the binary switching device at a repetitionrate of at least approximately 10 cycles per second, more preferably atleast approximately 20 cycles per second, and most preferably at leastapproximately 25 cycles per second.

[0008] An electrochromic rearview mirror assembly, according to thevarious aspects of the invention, may additionally include otherfunctions of the rearview mirror including a display which displays thevehicle heading, determined by a compass, the outdoor temperature,determined by an outdoor temperature sensor, or both the vehicle headingand outdoor temperature. The digital controller, which is preferably amicrocomputer, may additionally control the intensity of the display.The intensity of the display may be controlled as a function of lightlevels around the vehicle. Additionally, in particular embodiments, thedisplay may be positioned behind the electrochromic cell wherein thedisplay is viewed through the electrochromic cell. In such embodiments,the microcomputer may additionally adjust the intensity of the displayas a function of the reflectance level of the reflective element. Inthis manner, the display, as perceived by the driver, does not vary inintensity as the reflectance level of the reflective element changes.However, the intensity of the display may be adjusted to accommodate thephysiological response of the driver's eyes.

[0009] These and other objects, advantages and features of thisinvention will become apparent upon review of the followingspecification in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a side elevation of a vehicle having an electrochromicrearview mirror assembly, according to the invention;

[0011]FIG. 2 is a side elevation of an electrochromic rearview mirrorassembly, according to the invention, represented schematically toillustrate components of the electronic control thereof;

[0012]FIG. 3 is a block diagram of the electronic control in FIG. 2;

[0013]FIG. 4 is a schematic diagram of the electronic control in FIG. 3;

[0014]FIG. 5 is a diagram of a pulsed drive signal;

[0015]FIG. 6 is the same view as FIG. 5 of an alternative embodimentthereof;

[0016]FIG. 7 is a software flowchart of a control algorithm for anelectrochromic rearview mirror assembly;

[0017]FIG. 8 is a table of source adjustment steps; and

[0018]FIG. 9 is a schematic diagram of an alternative electroniccontrol.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0019] Referring now specifically to the drawings. and the illustrativeembodiments depicted therein, a vehicle 9 is illustrated as having anelectrochromic rearview mirror assembly 11 (FIGS. 1-3). Although theinvention is illustrated in an interior rearview mirror assembly, theinvention could be equally applied to exterior rearview mirrorassemblies as well as to an entire electrochromic rearview mirrorsystem. Electrochromic rearview mirror 11 includes an electronic control12 and a variable reflectance electrochromic reflective element 18having an electrochromic cell 15 and a reflective surface 17.Electrochromic element 18 may be of any known type, such as disclosed inU.S. Pat. No. 4,902,108 issued to Byker; commonly assigned U.S. Pat. No.5,140,455 issued to Varaprasad et al; commonly assigned U.S. patentapplication Ser. No. ______ filed Mar. 27, 1997, by Varaprasad et al.entitled ELECTROCHROMIC POLYMERIC SOLID FILMS, MANUFACTURINGELECTROCHROMIC DEVICES USING SUCH SOLID FILMS, AND PROCESSES FOR MAKINGSUCH SOLID FILMS AND DEVICES (continuation-in-part of application Ser.No. 08/406,663) (Attorney Docket No. 690.3 CIP C1 CIP), and commonlyassigned U.S. patent application Ser. No. 08/429,643 filed Apr. 27,1995, by Varaprasad et al., entitled ELECTROCHROMIC MIRRORS AND DEVICES,the disclosures of which are hereby incorporated herein by reference.Electrochromic element 18 dims to a partial reflectance level inresponse to a drive signal applied thereby.

[0020] Electronic control 12 includes a drive circuit 13 which receivesinputs from a substantially rearwardly directed light sensor 20 and froma substantially forwardly directed light sensor 22 and provides outputsto control the reflectance level of electrochromic partial reflectiveelement 18. Light sensors 20, 22 make up a light sensor combination 28which provides input to a microcomputer U2 (FIGS. 3 and 4). A digitalcontroller, such as microcomputer U2, is a low current source, typicallyin microamps to less than about 25 milliamperes, which provides logiclevel outputs to a high current source 40 which applies direct currentpulses derived from vehicle ignition voltage, typically between 8 VDCand 18 VDC with 12 VDC nominal, to the electrochromic cell source 40which has a current capability of at least about 50 milliamperes,preferably of at least about 100 milliamperes, and most preferably of atleast about 200 milliamperes. As will be described in more detail below,the duty cycle of these pulses establishes the partial reflectance levelof reflective element 18. As schematically illustrated in FIG. 3, ablanking logic signal, which is typically pulse-width modulated, isoutput at 26 b from microcomputer U2 based on the condition of highwayglare light and ambient light conditions around the vehicle as detectedby the light sensor combination 28. Such light sensor combinations areconventional and are described in U.S. Pat. No 4,917,477 issued toBechtel et al., U.S. Pat. No. 4,793,690 issued to Graham et al., andU.S. Pat. No. 3,601,614 issued to Platzer, Jr., the disclosures of whichare hereby incorporated herein by reference. The logic signal output at26 is input to high current source 40. The amplitude of the outputsignal from high current source 40 is variable within a narrow rangeestablished close to, and preferably constrained from significantlyexceeding, the maximum voltage tolerable for a sustained period byelectrochromic cell 15.

[0021] The amplitude of the pulsed output signal from source 40 can beadjusted by microcomputer U2 over outputs 26 c and 26 d as a function ofthe voltage developed across electrochromic cell 15. The developedvoltage is sensed over a line 48 extending from a terminal 44 of thecell to an input of microcomputer U2. Such electrochromic cellstypically develop a voltage, which is a back Electromotive Field (EMF)upon application of an external voltage thereto, and temporarily retainthat back voltage, or back EMF, even when the external voltage potentialis removed and the cell is open-circuited. Also, for solution-phasesingle compartment, self-erasing electrochromic mirror elements commonlyused commercially today, the maximum voltage tolerable for a sustainedperiod is in the 1.0 V to 2.0 V range, typically less than 1.5 V andmost typically about 1.4 V. For solid-film electrochromic devices thatutilize a layer, such as a tungsten oxide thin film layer, the maximumvoltage tolerable for a sustained period is in the 1.0 V to the 3.0 Vrange, typically in the 1.3 V to 1.5 V range. Usually, application of avoltage much in excess of such maximum tolerable voltage to theelectrochromic cell for a sustained period, typically at least severalseconds, may cause change to the electrochromic medium in theelectrochromic medium in the electrochromic cell.

[0022] In the illustrated embodiment, microcomputer U2 is marketed byToshiba Corporation of Japan under Model No. TMP87C4008, but could beimplemented by microcomputers marketed by other manufacturers.Microcomputer U2 includes a plurality of inputs 24 and a plurality ofoutputs 26. Outputs 26 are tri-state outputs which are capable ofassuming a low state in which the output is pulled to ground, a neutralhigh impedance state in which the output is effectively open-circuited,and a high state in which the output is driven to a positive, ornegative, DC voltage. Inputs 24 a and 24 b are connected with lightsensor combination 28 made up of rearward-directed light sensor 20 andforward-directed light sensor 22 electrically connected in series witheach other and with a resistor RA7. This series circuit is connectedbetween a positive source of voltage (5V) and ground. Input line 24 a isconnected with a junction, or node, 30 between light sensors 20 and 22.Input 24 b is connected with a junction, or node, 32 between rearwardlight sensor 20 and resistor RA7. As disclosed in detail in commonlyassigned co-pending application Ser. No. 08/768,193, filed Dec. 17,1996, by Schierbeek et al., for an AUTOMATIC REARVIEW MIRROR SYSTEM WITHAUTOMATIC HEADLIGHT ACTIVATION, the disclosure of which is herebyincorporated herein by reference, the voltage at node 30 is used toestablish a reflectance level of electrochromic reflective element 18.The voltage at junction 32 is representative of the overall light levelsurrounding vehicle 9 and is used in a manner which will be describedbelow.

[0023] In the illustrative embodiment, electronic control 12 includes adisplay 34 which is driven by a display driver U3. In the illustratedembodiment, display 34 is marketed by National Electric Corporationunder Model No. FIP2QMBS and driver U3 is marketed by Allegro underModel No. UNC5812EPF, although other commercially available componentsmay also be used. Display 34 may be positioned behind electrochromiccell 15 of reflective element 18 and viewed by the driver through theelectrochromic cell, as disclosed in U.S. Pat. No. 5,285,060, issued toLarson et al., for a DISPLAY FOR AUTOMATIC REARVIEW MIRROR, thedisclosure of which is incorporated herein by reference. Alternatively,display 34 may be positioned on a lip portion of housing 14 belowreflective element 18 or on any other portion of the housing visible tothe driver as illustrated in commonly assigned patent application Ser.No.______ filed Feb. 12, 1997, by Schofield et al., for a VEHICLE BLINDSPOT DETECTION DISPLAY SYSTEM (Attorney Docket No. DON01 P-651), thedisclosure of which is hereby incorporated herein by reference.Alternatively, display 34 could be in the form of a heads-up displayprojected from housing 14 on the vehicle windshield 16.

[0024] Electronic control 12 may additionally include a heading sensor,or compass, 36 which produces outputs 38 indicative of the heading ofthe vehicle. Such heading sensor may be of the magneto-resistive type,such as disclosed in commonly., assigned U.S. Pat. No. 5,255,442, issuedto Schierbeek et al., for a VEHICLE COMPASS WITH ELECTRONIC SENSOR, ormay be of the magneto-inductive type, such as disclosed in commonlyassigned provisional patent application Serial No. 60/027,996, filedOct. 9, 1996, by Domanski for an ELECTRONIC COMPASS, the disclosures ofwhich are hereby incorporated herein by reference, or may be of theflux-gate type, or may be of the magneto-capacitive type. The heading ofthe vehicle detected by heading sensor 36 is encoded on outputs 38,decoded by driver U3 and displayed by display 34.

[0025] Microcomputer U2 includes an output 26 a, which controls theintensity of display 34. In the embodiment illustrated in FIG. 4,microcomputer U2 provides a signal on line 26 a which adjusts theintensity of display 34 according to the light level around the vehicleas provided on input 24 b. As disclosed in the Larson et al. '060patent, microcomputer U2 reduces the intensity of display 34 during lowlight levels in order to avoid dazzling the driver. During high lightlevels, microcomputer U2 increases the intensity of display 34 in orderto make the display more discernable to the driver. If display 34 ispositioned behind cell 15, wherein the output of the display is viewedthrough cell 15, microcomputer U2 additionally adjusts the intensity ofdisplay 34 as a function of the light transmission level of cell 15 inorder to compensate for attenuation of light transmission by the cell.This is accomplished by increasing the intensity of display 34 for lowerreflectance levels of electrochromic reflective element 18 resultingfrom coloration of cell 15 to a lower light transmission level asdisclosed in the Larson et al. '060 patent.

[0026] Electronic control 12 includes a source 40 for supplying directcurrent energy to color cell 15 to a partial light transmission level.Source 40 is made up of a voltage divider composed of resistors R4 andR7 connected in series between a ⁺5 volt source and ground. A node 42 ofthe voltage divider is supplied to a Darlington transistor pair Q1 andQ2 which apply a DC voltage to a first terminal 44 of cell 15. Anotherterminal 46 of cell 15 is connected with ground. As is known in the art,the voltage applied to the base of transistor Q1 is decreased by twoforward base-emitter drops and applied to terminal 44. In theillustrated embodiment, resistors R4 and R7 are selected to apply anominal voltage of approximately 1.35 to 1.4 volts to cell 15. Thisrange may vary depending upon the particular type of electrochromiccell. A transistor Q3 is connected directed across terminals 44 and 46.When a voltage is applied to the base of transistor Q3 sufficient todrive Q3 into saturation, an essentially short circuit is applied acrosscell 15 which rapidly removes at least a portion of the charge appliedto the cell. Microcomputer U2 controls tile states of transistors Q1-Q3in a binary fashion, wherein each transistor is either conducting oropen-circuited, and adjusts the output level of source 40, bycontrolling outputs 26 b, 26 c, 26 d and 26 e in a manner which will bedescribed below.

[0027] Output 26 b, when in a neutral high impedance state does, notsubstantially affect the voltage at node 42 whereby the voltage at node42 is established solely by resistors R4 and R7. This voltage drivestransistors Q1 and Q2 into conduction and applies a voltage level,dependent on the voltage of source 40 to cell 15. When output 26 b isdriven to a low state, the voltage at node 42 is decreased to a level atwhich transistors Q1 and Q2 become open-circuited and no current issupplied to cell 15. In the illustrated embodiment, output 26 b is notdriven to a high state, although, in other embodiments, the high-outputstate may be useful if appropriate adjustments are made to the circuit.

[0028] Outputs 26 c and 26 d serve as “fine” and “coarse” adjustments,respectively, to the voltage level at node 42. When output 26 d, whichis the “coarse” adjustment, is in a neutral high impedance state, theoutput has no effect on the voltage at node 42. When output 26 d isdriven to a low state, a resistor R6 is placed in parallel with resistorR7, which decreases the voltage at node 42. When output 26 d is drivento a high state, resistor R6 is essentially in parallel with resistor R4which increases the voltage at node 42. Likewise, when “fine” adjustmentoutput 26 c is neutral, it has no effect on the voltage at node 42. Whenoutput 26 c is driven low, a resistor R5 is placed in parallel withresistor R7 which decreases the voltage at node 42, and when output 26 cis driven high, resistor R5 is placed in parallel with resistor R4,which increases the voltage level at node 42. Because resistor R6 has alow resistance value than resistor R5, the effect of output 26 d isgreater than that caused by output 26 c.

[0029] Output 26 e controls the conductive state of transistor Q3. Whenoutput 26 e is in a neutral high impedance state, there is no basedriven to transistor Q3 and Q3 is open-circuited. When output 26 e isdriven high, transistor Q3 is driven to a conductance state, which, aspreviously set forth, places a substantially short circuit across cell15 which, as is known in the art, removes at least a portion of thecharge on cell 15.

[0030] Terminal 44 of cell 15 is interconnected through a line 48 and aresistor R19 to an input 24 c of microcomputer U2. This provides aninput to microcomputer U2, which represents the voltage across cell 15.This voltage is buffered by resistor R19 in order to avoid damage tomicrocomputer U2 by spurious voltages on the cell. A capacitor C19maintains the voltage level at input 24 c against fluctuation duringeach analog-to-digital conversion carried out internally bymicrocomputer U2. As is known in the art, the voltage level across cell15 is generally, but not necessarily precisely, related to the degree ofcoloration of light transmission level of cell 15. In this manner,microcomputer U2 is provided with information concerning the generalreflectance level of reflective element 18. This information iscollected and used in a manner which will be set forth below.

[0031] Electronic control 12 additionally includes a switch S2 which isdriver-operable in order to switch the rearview mirror between an“automatically controlled” state in which the reflectance level ofreflective element 18 is controlled and an “off” state in which thereflectance level of reflective element 18 is not controlled. One wiperof switch S2 is connected with an 8.0 volt source and is selectivelyconnectable with a line 50 which supplies voltage to transistors Q1 andQ2. Therefore, when in the position illustrated in FIG. 3, no voltage issupplied to the transistors, and the cell remains in a high reflectancestate. Additionally, switch S2 includes a wiper which is connectedthrough a line 52 connected with terminal 44. When in the positionillustrated in FIG. 3, terminal 44 is directly connected with groundwhich rapidly bleaches the cell to a high reflectance condition.Alternatively, polarity to the cell could be reversed to provide a powerbleach. Electronic control 12 additionally includes a reverse-inhibitinput 24 d which causes microcomputer U2 to force output 26 b to a lowstate and output 26 e to a high state and thereby bleaches cell 15 whenthe vehicle is in reverse gear. Electronic control 12 additionallyincludes an indicator D2, which, when actuated, indicates to the driverthat the control is actively controlling the reflectance level ofreflective element 18.

[0032] Electronic control 12 may additionally, optionally, include aseries of resistors R20-R23 which are connected as illustrated asvoltage dividers in order to supply inputs 24 e and 24 f tomicrocomputer U2. Inputs 24 e and 24 f establish the sensitivity ofmicrocomputer U2 to signals received from light sensor combination 28and may be changed in value for different vehicle configurations inwhich rearview mirror 11 is provided. Sensitivity settings mayadditionally be stored in Erasable Electrically Programmable Read-OnlyMemories (EE-PROM) and thereby electrically selectable for the vehicletype in which rearview mirror 11 is positioned. Additionally, suchEE-PROM (not shown) may be used to provide characterization data oflight sensors 20 and 22 in order to allow different light sensors to beutilized and to make compensation for the different characteristics ofeach light sensor for use by microcomputer U2. In known electrochromicdrive circuits, it is necessary to adjust the values of resistors RA7and RA15 in order to compensate for variations in light sensors 20, 22.This is typically accomplished either by providing a variablepotentiometer to make production-line calibration adjustments or bycharacterizing each light sensor and matching up suitable values ofresistors RA7 and RA15. Both procedures are cumbersome . With the use ofan EE-PROM, the characterization data of the light sensors can be storedin the EE-PROM and used to compensate for variations in light sensorcharacteristics. For example, variations which previously would havebeen compensated for by selecting the value of resistor RA7, can becompensated for by internal set point variations in the algorithm usedby microcomputer U2. Variations which previously would have beencompensated for by selecting the value of resistor RA15 can becompensated for by providing a resistor between a part of microcomputerU2 and a terminal of resistor RA15, with the microcomputer selecting, ahigh output state for that part to lower the resistance value ofresistor RA15, or a neutral state to not affect the resistance ofresistor RA15. Microcomputer U2 may additionally be provided withlinearization data, whereby the voltage level at node 30, which variesnon-linearly for various light levels sensed by sensors 20 and 22, maybe interpreted linearly for the purpose of producing a drive signal todrive cell 15 to a particular reflectance level.

[0033] In operation, microcomputer U2 switches output 26 b between aneutral high impedance state and a low state in order to pulsetransistors Q1 and Q2 together and thereby apply a pulsed direct currentto cell 15. In contrast to conventional electrochromic element drivecircuits which supply a steady DC voltage level in order to control thereflectance level of reflective element 18, microcomputer U2 controlsthe reflectance level of the reflective element by varying the dutycycle of the pulsed signal applied to cell 15. Such a pulsed signal P isillustrated in FIG. 5 and is shown as having an approximately 50 percentduty cycle. As the duty cycle decreases in percent on-time versesoff-time for transistors Q1 and Q2, the current supply to cell 15decreases and thereby the reflective element assumes a high reflectancecondition. In contrast, as the duty cycle of signal P increases, byswitching transistors Q1 and Q2 on for a greater percentage of time ascompared to the off period of these transistors, a greater amount ofcharge is supplied to cell 15 and thereby the cell colors electrochromicreflective element 18 assumes a lower reflectance level.

[0034] Microcomputer U2 is capable of providing a pulsed DC supply tocell 15 according to a variable duty cycle, and thereby is capable ofestablishing a particular reflectance level for reflective element 18,by relying upon the natural tendency of cell 15 to discharge itselfduring periods when current is not being supplied by transistors Q1 andQ2. In the illustrated embodiment, discharge of cell 15, when not beingcharged through transistors Q1 and Q2, is enhanced by transistor Q3which actively discharges cell 15 between pulses of DC supplied bytransistors Q1 and Q2. Thus, by reference to FIG. 4, during period A,transistors Q1 and Q2 are driven in order supply a DC level, which isillustrated as being positive but could also be negative, to the cell.During period B, after microcomputer U2 has turned off transistors Q1and Q2, transistor Q3 is driven to a conductive state in order torapidly discharge cell 15. This provides superior control over theresponse of cell 15 to the variable duty cycle pulse train supplied fromsource 40 under the control of microcomputer U2 than would be achievedby control of only the application of the source to the cell.Microcomputer U2 can vary the duty cycle of drive signal P from zeropercent (0%) to one hundred percent (100%).

[0035] Other factors besides the duty cycle of the drive signal Pinfluence the coloration of cell 15. For example, if the amplitude ofeach pulse is too high, the expected useful life of reflective element18 may decrease. If the amplitude of each pulse is too low, the cellwill not color to the desired level and thereby the reflectance level ofreflective element 18 will be too high. However, the ability to controlthe amplitude of each pulse in drive signal P is made difficult by theelectrical characteristics of cell 15 which vary both with temperatureand the degree of charge on the cell as well as tolerances in all of theelectrical components. By way of example, if cell 15 is completelydischarged, the cell will provide a greater electrical load and willtend to lower the amplitude of any pulse applied to the cell. However,if the charge on cell 15, which is represented by the voltage across thecell, is high relative to the amplitude of the pulse being applied, thecell will present a relatively small load on the pulse and the amplitudeof the pulse will not be lowered. In order to provide control over theamplitude of the DC pulses applied to cell 15, electronic control 12includes a feedback loop through microcomputer U2 utilizing input 24 cto monitor the voltage across cell 15 through line 48 which connectswith terminal 44 of the cell. This input monitors the voltage across thecell produced by each pulse. If the voltage is too low, the amount ofdrive applied to the next pulse is increased. If the voltage producedacross the cell is too high, thereby potentially reducing the lifetimeof the cell, microcomputer U2 lowers the amplitude of the next pulse. Ifthe voltage across cell 15, as sampled by input 24 c is within adesirable range, then microcomputer U2 keeps the same amplitude for thenext pulse.

[0036] As can be seen by reference to FIG. 5, microcomputer U2 monitorsthe voltage across cell 15 by sampling the voltage on the cell at pointS which is selected to be at the end of the applied pulse. At point S,the voltage produced across cell 15 by that pulse will have presumablystabilized so that the measured voltage is assume to be an accuraterepresentation of the voltage across the cell. Of course, it may bepossible to monitor the voltage across the cell at other points on thepulse or to measure the amplitude at several points and average theresults. By reference to FIG. 5, the pulse sampled at S1 is determinedby microcomputer U2 to produce a voltage across cell 15 which is belowthe range R established for the particular cell. Therefore,microcomputer U2 increases the amplitude of source 40 for producing thenext pulse, the effect on cell 15 of which is sampled at S2. Because inthe illustration, microcomputer U2 determines that the sampled voltageacross cell 15 at S2 is within range R, no adjustment is made to theamplitude of source 40 for the next pulse. When a sample S3 is made ofthe voltage across cell 15 during the next pulse, the sampled voltage isgreater than range R which causes microcomputer U2 to lower theamplitude of source 40 for producing the next pulse which is sampled atS4.

[0037] As set forth above, microcomputer U2 is capable of adjusting theamplitude of source 40 at node 42 and thereby the amplitude of the pulseapplied to the cell by controlling the states of outputs 26 c and 26 d.This is accomplished digitally utilizing the port settings illustratedin FIG. 8. By reference to FIG. 8, eight steps of voltage adjustment areavailable to the microcomputer by selecting a Most Significant Bit (MSB)as the “coarse” output 26 d and a Least Significant Bit (LSB) as “fine”output 26 c. By reference to FIG. 8, if no change is required in thevoltage level of source 40, a step number 4 is selected which provides aneutral high impedance state on outputs 26 c and 26 d. In order todecrease the voltage level of source 40, a lower step number isselected. The greatest reduction of voltage is achieved by step number 0in which ports 26 c and 26 d are both driven to low states which arerepresented by a 0. Conversely, if microcomputer U2 wishes to raise thevoltage of source 40, a step higher than 4 is selected, with step 8being the greatest increase.

[0038] Electronic control 12 operates as follows. Periodicallymicrocomputer U2 monitors the voltage at node 30 utilizing input 24 awhich includes an internal Analog-to-Digital (A/D) converter.Microcomputer U2 computes the Pulse With Modulation percentage (PWM%)corresponding to the sensed light level according to formula 1:

PWM%=G(C _(S) −V _(A/D))  (1)

[0039] where:

[0040] G=Gain (a constant);

[0041] C_(s)=Voltage at the start of color of the cell; and

[0042] V_(A/D)=A/D voltage at input 24 a.

[0043] Ideally, PWM% will equal to 0 when the A/D voltage is equal tothe voltage at which it is desired to begin coloration of the cell 15.As the voltage on node 30 decreases, the PWM% increases until the PWM%equals 100 percent. If formula 1 yields a negative value, microcomputerU2 sets the PWM% to 0. Any values greater than 100 PWM% are kept at 100PWM%.

[0044] Once microcomputer U2 determines the PWM% utilizing formula 1, acontrol algorithm 55 is carried out (FIG. 7). The voltage across cell 15is measured at 60 at point S and it is determined at 62 whether thesample voltage is below the value of ECMIN, which, in the illustratedembodiment is set to 1.35 volts. If the voltage is less than ECMIN, itis determined at 64 whether the voltage adjustment has been set to themaximum value. If not, the port settings are incremented at 66 and thenew port settings are applied to source 40 at 68 in order to adjust theamplitude of the next pulse supplied to cell 15. If it is determined at64 that the port setting is at a maximum value, then no additionaladjustment is possible.

[0045] If it is determined at 62 that the sample voltage across the cellis not less than the minimum, it is determined at 70 if the samplevoltage is greater than ECMAX. In the illustrated embodiment, ECMAX isapproximately 1.40 volts. If it is determined at 70 that the sample cellvoltage is greater than ECMAX, then a similar adjustment is made to theamplitude of source 40 for the next pulse, except in the oppositedirection, as follows. At 72, it is determined whether the minimumvoltage adjustment has been achieved. If not, the setting is decrementedat 74 and the new port settings are outputted at 76 in order to adjustdownwardly the amplitude of source 40. If it is determined at 72 thatthe voltage adjustment value is at a minimum, then a parameter MAXDC isdecreased by a value, such as 10 percent at 78. MAXDC is a maximum dutycycle that microcomputer U2 will apply to cell 15 and is used in orderto prevent prolonged over-stimulation of the cell. By decreasing thevalue of MAXDC, temporary over-voltage pulses are applied to the cellaccording to a lower duty cycle and thereby reducing the effect of theover-voltage condition on the cell.

[0046] If it is determined at 70 that the sampled voltage across thecell is not greater than ECMAX, then the sample voltage is within thedesired range R. It is then determined at 80 whether the value of MAXDCis less than 100 percent. If it is determined at 80 that the value ofMAXDC is less than 100 percent, the value of MAXDC is increased at 82 by10 percent. This allows microcomputer U2 to drive the cell at a higherduty cycle, closer to, or equal to, 100 percent, provided that thevoltage produced on the cell is within range R.

[0047] Alternatively, a drive signal P′, having a variable duty cycle,may be utilized to drive cell 15 to the desired reflectance level ofreflective element 18 (FIG. 6). Drive signal P′ includes three distinctperiods. During period A1, transistors Q1 and Q2 are in conduction whichapplies a current from source 40 to charge cell 15. In period Bmicrocomputer U2 opens transistors Q1 and Q2 and closes transistor Q3 inorder to drain the charge on cell 15. During a period A2, betweenperiods A1 and B, all transistors Q1-Q3 are open-circuited. At the endof period A1, microcomputer U2 samples the voltage across cell 15 at apoint represented by S_(ON). During period A2, when cell 15 is beingneither stimulated nor drained, a second sample S_(OFF) is made bymicrocomputer U2 of the voltage across cell 15. This sample made duringS_(OFF) can be utilized by microcomputer U2 in order to obtain anapproximation of the level of coloration of cell 15. This informationmay then be used by microcomputer U2 in order to determine, for example,if the reflectance level desired of cell 15 is significantly greaterthan the present approximate reflectance level of cell 15. Thisinformation can be used in many ways. For example, if the sample takenat S_(OFF) indicates that cell 15 is in a high light transmissioncondition, whereby reflective element 18 is at a high reflectance level,and it is determined that the reflectance level of the reflectiveelement must be significantly decreased, then microcomputer U2 maytemporarily, intentionally, apply a voltage which is greater than ECMAXand/or a duty cycle which is greater than that which corresponds to theselected reflectance level, in order to increase the rate of colorationof cell 15 utilizing the principles disclosed in commonly owned U.S.Pat. No. 5,220,317, issued to Lynam et al., for an ELECTROCHROMIC DEVICECAPABLE OF PROLONGED COLORATION, the disclosure of which is herebyincorporated herein by reference. Likewise, if it is determined atS_(OFF) that the transmission level of cell 15 is very low, whereby thereflectance level of reflective element 18 is low, and it is desired torapidly increase the reflectance level of the element, microcomputer U2may intentionally apply a voltage and/or duty cycle which is temporarilybelow target to more quickly achieve the desired reflectance level.

[0048] An alternative electronic control 12′ is illustrated in FIG. 9,which includes an outdoor temperature sensor 86 utilized to supply aninput to a microcomputer U4 for display on a display 34′. Additionally,electronic control 12′ provides control over indicators D4 bymicrocomputer U4. Indicators D4, which are red and green in color, mayprovide an indication to the driver that the electrochromic controlfunction is operating according to high sensitivity (green), operatingaccording to a low sensitivity (red), or is completely off. In thisembodiment, sensitivity of the drive circuit is user selectableutilizing soft-touch switches S1 and S2 mounted on housing 14.Electronic control 12′ controls the intensity of display 34′ accordingto light levels surrounding the vehicle as determined by the voltage atthe light sensor (not shown in FIG. 9). Additionally, microcomputer U4controls the intensity of indicators D4 according to light levelssurrounding the vehicle. In this manner, the light levels of theindicators, as well as that of the display, are controlled according tothe physiological condition of the driver responding to light levelssurrounding the vehicle.

[0049] It has been determined that the repetition rate of the pulses indrive signals P and P′ (FIGS. 5 and 6) should be above approximately 10cycles per second (Hz) to avoid any significant perception of flickeringof the reflectance level of the reflective element. While any repetitionrate greater than 10 Hz is desirable, a repetition rate aboveapproximately 20 Hz is preferred and a repetition rate above 25 Hz ismost preferred. For example, for electrochromic mirrors which color from65% to 20% reflectivity in a time period of less than about 4 seconds,it has been found that a repetition rate of 20 Hz produced noperceivable flicker to a human observer.

[0050] Thus, it is seen that the present invention utilizes digitallogic control, which is incorporated into vehicle functions, such asvehicle heading display and temperature sensing and display, in order toperform functions which require handling of a significant amount ofcurrent while maintaining a high degree of control over applied voltageto the electrochromic cell. By controlling the reflectivity level of themirror, or mirrors, utilizing the duty cycle of a Pulse-Width Modulated(PWM), or a blanking, signal, the information processing capabilities ofdigital logic may be applied to the unique problem of controlling anelectrochromic rearview mirror element. Although the invention isillustrated as implemented with a microcomputer, other digital logiccircuits, such as programmable arrays and the like, may be utilized.

[0051] The present invention can be used with interior rearview mirrorassemblies equipped with a variety of features, such as a high/low (ordaylight running beam/low) headlamp controller, a hands-free phoneattachment, a video camera for internal cabin surveillance and/or videotelephone function, seat occupancy detection, a cellular phonemicrophone, map-reading lights, compass/temperature display, fuel leveland other vehicle status display, a trip computer, an intrusiondetector, contacting rain sensors, non-contacting rain sensors, and thelike. Such features can share components and circuitry with theelectrochromic mirror circuitry and assembly so that provision of theseextra features is economical.

[0052] The digital electrochromic mirror system of this invention can beutilized in a vehicle that utilizes a car area network, such as isdescribed in Irish Patent Application No. 970014 entitled A VEHICLEREARVIEW MIRROR AND A VEHICLE CONTROL SYSTEM INCORPORATING SUCH MIRROR,filed Jan. 9, 1997, the disclosure of which is hereby incorporated byreference herein and can be a node of that car area network, or, whenmultiplexing is used, such as is disclosed in U.S. patent applicationSer. No. 08/679,631 entitled VEHICLE MIRROR DIGITAL NETWORK ANDDYNAMICALLY INTERACTIVE MIRROR SYSTEM, by O'Farrell et al., filed Jul.11, 1996, the disclosure of which is hereby incorporated by referenceherein. Also, given that an interior electrochromic mirror canoptionally be equipped with a myriad of features (such as map lights,reverse inhibit line, headlamp activation, external temperature display,remote keyless entry control, and the like), it is useful to equip suchassemblies with a standard connector (for example, a 10-pin parallelconnector) so that a common standard wiring harness can be providedacross an automaker's entire product range. Naturally, multiplexingwithin the vehicle can help alleviate the need for more pins on such aconnector or allow a given pin or set of pins control more than onefunction.

[0053] Using the concepts of the present invention, a drive voltage at,or close to, the maximum voltage tolerable by the electrochromic mirrorelement (ECMAX) can be selected (for example, 1.4 V) and, using thisvoltage ECMAX as a modulated, or a blanking, signal, the reflectivity ofthe electrochromic reflective element can be controlled to any partialreflectance level within its range of reflectance levels from itsmaximum (bleached) reflectivity to its minimum (fully dimmed)reflectivity by, for example, varying the duty cycle of the modulatedsignal. The continuously variable control of mirror reflectivity, alsoreferred to as “gray-scale control,” is achieved by varying the dutycycle of the blanking signal, preferably by pulse-width modulation.

[0054] Although illustrated as applied to control of an electrochromicmirror element, the principles of the invention can be applied to otherdevices including windows, glazings, contrast enhancement filters,sunroofs, and the like.

[0055] Changes and modifications in the specifically describedembodiments can be carried out without departing from the principles ofthe invention which is intended to be limited only by the scope of theappended claims, as interpreted according to the principles of patentlaw including the doctrine of equivalents

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An electrochromicrearview mirror assembly for a vehicle comprising: an electrochromicreflective element having an electrochromic cell wherein said reflectiveelement colors to a partial reflectance level in response to a drivesignal applied to said electrochromic cell; and a drive circuit whichapplies a pulsed drive signal to said electrochromic cell in order toestablish said partial reflectance level of said reflective element saiddrive circuit controlling said partial reflectance level at least as afunction of the duty cycle of said pulsed drive signal.
 2. Theelectrochromic rearview mirror assembly in claim 1 wherein said drivecircuit applies a direct current signal to said electrochromic cellduring one portion of each duty cycle and sinks a direct current signalfrom said electrochromic cell during another portion of each duty cycle.3. The electrochromic rearview mirror assembly in claim 2 wherein saiddrive circuit controls the amplitude of said pulsed drive signal.
 4. Theelectrochromic rearview mirror assembly in claim 3 wherein said drivecircuit monitors the voltage across said electrochromic cell and adjustsat least one of the direct current signal applied during subsequentpulses and the duty cycle of the pulsed drive signal as a function ofthe voltage across the electrochromic cell.
 5. The electrochromicrearview mirror assembly in claim 4 wherein said drive circuit monitorsthe voltage across said electrochromic cell sampling the voltage nearthe end of said one portion.
 6. The electrochromic rearview mirrorassembly in claim 2 wherein said drive circuit monitors the voltageacross said electrochromic cell between said portions of each duty cyclewhen said drive circuit is neither applying a direct current signal tosaid cell nor sinking a direct current signal from said cell in order tocontrol at least one of the amplitude and the duty cycle of said pulseddrive signal.
 7. The electrochromic rearview mirror assembly in claim 1wherein said drive circuit monitors the voltage across saidelectrochromic cell and adjusts at least one of the amplitude of saiddrive signal during subsequent pulses and the duty cycle of the pulseddrive signal as a function of the voltage across the electrochromiccell.
 8. The electrochromic rearview mirror assembly in claim 1 whereinsaid pulsed drive signal has a repetition rate of at least approximately10 cycles per second.
 9. The electrochromic rearview mirror assembly inclaim 8 wherein said pulsed drive signal has a repetition rate of atleast approximately 20 cycles per second.
 10. The electrochromicrearview mirror assembly in claim 1 wherein said drive circuit comprisesa microcomputer.
 11. The electrochromic rearview mirror assembly inclaim 10 further including a display and at least one of a compass andan outdoor temperature sensor, wherein said display displays vehicleheading, outdoor temperature, or both vehicle heading and outdoortemperature.
 12. The electrochromic rearview mirror assembly in claim 11wherein said microcomputer controls the intensity of said display. 13.The electrochromic rearview mirror assembly in claim 12 including alight sensor which senses light level around the vehicle, wherein saidmicrocomputer adjusts the intensity of said display in response to saidlight sensor as a function of the light level around the vehicle. 14.The electrochromic rearview mirror assembly in claim 13 wherein saiddisplay is positioned behind said electrochromic cell, wherein saiddisplay is viewed through said electrochromic cell and wherein saidmicrocomputer adjusts the intensity of said display as a function of thereflectance level of said reflective element.
 15. The electrochromicrearview mirror system of claim 14 wherein said drive circuit monitorsthe voltage across said electrochromic cell when said cell isopen-circuited in order to determine the reflectance level of saidreflective element.
 16. The electrochromic rearview mirror assembly inclaim 13 further including at least one indicator, wherein saidmicrocomputer adjusts the intensity of said at least one indicator inresponse to said light sensor as a function of the light level aroundthe vehicle.
 17. The electrochromic rearview mirror assembly in claim 12wherein said display is positioned behind said electrochromic cell,wherein said display is viewed through said electrochromic cell andwherein said microcomputer adjusts the intensity of said display as afunction of the reflectance level of said reflective element.
 18. Theelectrochromic rearview mirror assembly in claim 17 wherein said drivecircuit monitors the voltage across said electrochromic cell when saidcell is open-circuited in order to determine the reflectance level ofsaid reflective element.
 19. An electrochromic rearview mirror assemblyfor a vehicle, comprising: an electrochromic reflective element havingan electrochromic cell, wherein said reflective element colors to apartial reflectance level in response to a drive signal applied to saidelectrochromic cell; and a drive circuit which applies a drive signal tosaid electrochromic cell in order to establish the partial reflectancelevel of said reflective element, said drive circuit including a source,a digital controller, a switching device responsive to an output of saidcontroller for applying said source to said electrochromic cell and aninput of said controller responsive to the voltage developed across saidelectrochromic cell by said source, wherein said digital controllermonitors the voltage developed across said electrochromic cell, adjustssaid source, and closes and opens said switching device according to aparticular duty cycle in order to control said partial reflectancelevel.
 20. The electrochromic rearview mirror assembly in claim 19wherein said controller adjusts said source in order to cause saidvoltage developed across said electrochromic cell to extend beyond saidpredetermined range during transitions between substantially differentreflectance levels of said reflective element.
 21. The electrochromicrearview mirror assembly in claim 19 wherein said source is a voltagedivider and wherein said controller adjusts said source by connecting atleast one resistor in said voltage divider.
 22. The electrochromicrearview mirror assembly in claim 21 wherein said at least one resistorincludes at least two resistors of substantially different values andwherein said controller adjusts said source by connecting one or theother or both of said at least two resistors in said voltage divider.23. The electrochromic rearview mirror assembly in claim 22 wherein saidcontroller includes outputs operable between at least three differentstates, wherein said controller additionally adjusts said source byselecting a particular state for connecting one or the other or both ofsaid at least two resistors in said voltage divider.
 24. Theelectrochromic rearview mirror assembly in claim 19 including anotherswitching device responsive to an output of said controller for drainingcharge from said electrochromic cell, wherein said controlleralternatingly closes said switching devices according to said particularduty cycle.
 25. The electrochromic rearview mirror assembly in claim 24wherein said controller keeps both said switching devices open during aportion of said duty cycle in order to sample residual voltage acrosssaid electrochromic cell and thereby determine the reflectance level ofsaid reflective element.
 26. The electrochromic rearview mirror assemblyin claim 19 wherein said controller samples said input when said binaryswitching device is closed in order to monitor the voltage developedacross said electrochromic cell by said source.
 27. The electrochromicrearview mirror assembly in claim 26 wherein said controller samplessaid input immediately prior to opening said switching device.
 28. Theelectrochromic rearview mirror assembly in claim 19 wherein said digitalcontroller closes and opens said switching device at a repetition rateof at least approximately 10 cycles per second.
 29. The electrochromicrearview mirror assembly in claim 28 wherein said digital controllercloses and opens said switching device a repetition rate of at leastapproximately 20 cycles per second.
 30. The electrochromic rearviewmirror assembly in claim 19 wherein said digital controller comprises amicrocomputer.
 31. The electrochromic rearview mirror assembly in claim30 further including a display and at least one of a compass and anoutdoor temperature sensor, wherein said display displays vehicleheading, outdoor temperature, or both vehicle heading and outdoortemperature.
 32. The electrochromic rearview mirror assembly in claim 31wherein said microcomputer controls the intensity of said display. 33.The electrochromic rearview mirror assembly in claim 32 including alight sensor which senses light level around the vehicle wherein saidmicrocomputer adjusts the intensity of said display in response to saidlight sensor as a function of light level around the vehicle.
 34. Theelectrochromic rearview mirror assembly in claim 33 wherein said displayis positioned behind said electrochromic cell, wherein said display isviewed through said electrochromic cell and wherein said microcomputeradjusts the intensity of said display as a function the reflectancelevel of said reflective element.
 35. The electrochromic rearview mirrorassembly in claim 34 wherein said drive circuit monitors the voltageacross said electrochromic cell when said cell is open-circuited inorder to determine the reflectance level of said reflective element. 36.The electrochromic rearview mirror assembly in claim 33 furtherincluding at least one indicator, wherein said microcomputer adjusts theintensity of said at least one indicator in response to said lightsensor as a function of light level around the vehicle.
 37. Theelectrochromic rearview mirror assembly in claim 32 wherein said displayis positioned behind said electrochromic cell, wherein said display isviewed through said electrochromic cell and wherein said microcomputeradjusts the intensity of said display as a function the reflectancelevel of said reflective element.
 38. The electrochromic rearview mirrorassembly in claim 37 wherein said drive circuit monitors the voltageacross said electrochromic cell when said cell is open-circuited inorder to determine the reflectance level of said reflective element. 39.An electrochromic rearview mirror assembly for a vehicle, comprising: anelectrochromic reflective element having an electrochromic cell, whereinsaid reflective element colors to a partial reflectance level inresponse to a drive signal applied to said electrochromic cell; and adrive circuit which applies a drive signal to said electrochromic cellin order to establish the partial reflectance level of said reflectiveelement, said drive circuit including a digital controller, a firstswitching device responsive to an output of said controller for applyinga source to said electrochromic cell and a second switching deviceresponsive to an output of said controller for draining charge from saidelectrochromic cell, wherein said controller alternatingly closes saidswitching devices according to a particular duty cycle in order tocontrol said partial reflectance level at least as a function of saidduty cycle, wherein said digital controller closes and opens saidswitching devices at a repetition rate of at least approximately 10cycles per second.
 40. The electrochromic rearview mirror assembly inclaim 39 wherein said digital controller closes and opens said switchingdevices at a repetition rate of at least approximately 20 cycles persecond.
 41. The electrochromic rearview mirror assembly in claim 39wherein said digital controller comprises a microcomputer.
 42. Theelectrochromic rearview mirror assembly in claim 41 further including adisplay and at least one of a compass and an outdoor temperature sensor,wherein said display displays vehicle heading, outdoor temperature, orboth vehicle heading and outdoor temperature.
 43. The electrochromicrearview mirror assembly in claim 42 wherein said microcomputer controlsthe intensity of said display.
 44. The electrochromic rearview mirrorassembly in claim 43 including a light sensor which senses light levelaround the vehicle wherein said microcomputer adjusts the intensity ofsaid display in response to said light sensor as a function of lightlevel around the vehicle.
 45. The electrochromic rearview mirrorassembly in claim 44 wherein said display is positioned behind saidelectrochromic cell, wherein said display is viewed through saidelectrochromic cell and wherein said microcomputer adjusts the intensityof said display as a function the reflectance level of said reflectiveelement.
 46. The electrochromic rearview mirror assembly in claim 45wherein said drive circuit monitors the voltage across saidelectrochromic cell when said cell is open-circuited in order todetermine the reflectance level of said reflective element.
 47. Theelectrochromic rearview mirror assembly in claim 44 further including atleast one indicator, wherein said microcomputer adjusts the intensity ofsaid at least one indicator in response to said light sensor as afunction of light level around the vehicle.
 48. The electrochromicrearview mirror assembly in claim 43 wherein said display is positionedbehind said electrochromic cell, wherein said display is viewed throughsaid electrochromic cell and wherein said microcomputer adjusts theintensity of said display as a function the reflectance level of saidreflective element.
 49. The electrochromic rearview mirror assembly inclaim 48 wherein said drive circuit monitors the voltage across saidelectrochromic cell when said cell is open-circuited in order todetermine the reflectance level of said reflective element.